Few products are sold by their manufacturer without some type of testing being conducted. Such testing may be as simple as manually ascertaining whether certain parts are securely affixed—or as complex as “stress testing.” In stress testing (or “stress screening” as it is sometimes called), products exhibiting “infant mortality” fail outright during the test. Or as the result of such testing, a product may evidence the likelihood of early failure in the operating environment.
Stress testing is most frequently employed with respect to products used in demanding applications and for which exceptionally-high reliability is required. Examples include products used on ground-traveling military equipment and products (e.g., electronic and electromechanical products) used in aircraft of essentially all types.
Stress testing may be carried out in any of several different ways. One type of test regimen involves imposing rapid, extreme changes in temperature upon the product. As an example, a test chamber may be used to change the temperature of a product between −70 degree C. and 200 degree C. over a period of, say, 5 minutes. Another type of test regimen involves using a test chamber to repetitively and dramatically change the relative humidity of air around a product. And humidity-based testing may also be accompanied by temperature-based testing and vice versa.
Yet another type of stress testing involves testing a product by subjecting it to vibrations of the type which might be encountered in actual product use. For example, U.S. Pat. No. 2,438,756 (Larsen) explains that the apparatus described therein is used to vibration-test electrical apparatus for airplanes, ships and the like. The unit described in U.S. Pat. No. 3,748,896 (Barrows) is said to be used for testing parts of a motor vehicle. And vibration testing is often conducted in conjunction with testing using another regimen, e.g., temperature.
Vibration testing is carried out by mounting the product to be tested upon some sort of platform or table and then vibrating the table using a rotating eccentric or a linear vibrator. Examples of devices used to create vibratory motion are shown in the Barrows patent and in U.S. Pat. Nos. 4,106,586 (Stafford) and 5,154,567 (Baker et al.).
In general, tables used to stress test products by application of vibration to such products are of two broad types, namely, flexible and rigid. An example of the former is disclosed in U.S. Pat. No. 4,735,089 (Baker et al.) and has a flexure member, i.e., a honeycomb structure, between two plates. An example of the latter is disclosed in FIGS. 12-14 of U.S. Pat. No. 5,412,991 (Hobbs) and has a rigid core plate between upper and lower plates.
While the prior art vibrator tables have been generally satisfactory for their intended purposes, they are not without disadvantages. In multi-degree freedom of testing vibration typically occurs on 3 axes, x, y and z. In a rigid table design there is significantly more vibration energy in one axis (typically the z-axis). A further disadvantage is that minimal energy is imparted in the 0-1000 Hz bandwidth. This rigid design has led to flexible table designs having a grouping of segmented plates, a composite of layers of different materials or combination of the two. These tables have acted to increase the energy in the 0-1000 Hz bandwidth but still did not normalize the energy between the three axes.
A disadvantage to known flexible tables is that they are highly expensive to manufacture since they utilize complex layering that is difficult to produce. Furthermore, such tables wear out or fail more rapidly since the materials used, or the interface adhering the materials together, deteriorate over time.
Therefore, an improved vibrator-driven table frame overcoming some of the problems and shortcomings of the prior art would be a distinct advance.